1. Field of the Invention
The present invention relates to a nitride semiconductor light emitting element, in particular, to a nitride semiconductor light emitting element with a light transmitting substrate having an upper surface provided with recesses and rises.
2. Description of the Background Art
Conventionally, for nitride semiconductor light emitting elements, sapphire (Al2O3) substrates, SiC substrates, GaN substrates, spinel (MgAl2O4) substrates, and the like have been used. Such nitride semiconductor light emitting elements are different from other compound semiconductor light emitting elements in that they employ the above-described substrates, which have a light transmitting property for the wavelength of emitted light. Since each of the substrates allows emitted light to transmit therethrough as such, light can be extracted not only from a surface of a semiconductor layer but also from a side surface of the substrate.
In a usual semiconductor light emitting element, a semiconductor layer is formed to have a thickness of several μm or so and a substrate has a thickness of approximately 100 μm. Hence, if light can be effectively extracted from a side surface of the substrate, light extraction efficiency in the semiconductor light emitting element can be improved significantly.
A nitride semiconductor layer has a refractive index of approximately 2.4, whereas, for example, a sapphire substrate has a refractive index of approximately 1.7. Therefore, when an interface between the nitride semiconductor layer and the sapphire substrate is irradiated with light entering at an incident angle greater than a critical angle, the light is totally reflected. The light thus reflected is repeatedly reflected in the nitride semiconductor layer, i.e., results in multiple reflections. During the multiple reflections, the light is absorbed and attenuated in an electrode or active layer formed on a surface of the semiconductor light emitting element. This results in weak intensity of light extracted externally.
A semiconductor light emitting element allowing for improved light extraction efficiency is disclosed in Japanese Patent Laying-Open No. 2005-317931. In the semiconductor light emitting element described in Japanese Patent Laying-Open No. 2005-317931, circular or triangular rises are formed on a surface of a sapphire substrate, and a nitride semiconductor layer is formed thereon. This scatters light having reached an interface between the sapphire substrate and the nitride semiconductor layer, and allows the light to come into the sapphire substrate. As a result, light is extracted at an increased rate from a side surface of the sapphire substrate to improve light extraction efficiency.
Meanwhile, a light emitting diode having very high light emitting efficiency is disclosed in Japanese Patent Laying-Open No. 2008-153634. In the light emitting diode described in Japanese Patent Laying-Open No. 2008-153634, rises each made from a dielectric formed of a material different from that of a substrate are formed on the substrate, and a nitride semiconductor layer is grown thereon. The rises each have a hexagonal shape, and are arranged in the form of a honeycomb structure when viewed in a planar view. The recesses and rises thus formed on the surface of the substrate scatter light to prevent multiple reflections thereof and accordingly improve light extraction efficiency.
Further, a semiconductor light emitting element allowing for improved light extraction efficiency and reduced operation voltage is disclosed in Japanese Patent Laying-Open No. 2005-259970. In the semiconductor light emitting element described in Japanese Patent Laying-Open No. 2005-259970, hexagonal recesses and rises are formed on a surface of a p type contact layer rather than on the substrate side to improve light extraction efficiency.
The conventionally used sapphire substrates, SiC substrates, spinel substrates, and the like are formed of a material different from that of the nitride semiconductor layer. Hence, when a nitride semiconductor layer is grown on each of such substrates, a multiplicity of defects such as threading dislocations occur due to mismatch in lattice constant or a difference in thermal expansion coefficient between the substrate and the nitride semiconductor.
A nitride semiconductor structure allowing for reduced threading dislocations in a nitride semiconductor film is disclosed in Japanese Patent No. 4016062. On the growth face of a substrate in the nitride semiconductor structure described in Japanese Patent No. 4016062, recesses and rises both made of the same material as that of the substrate are formed to facilitate lateral growth for improved crystal quality. The recesses are constituted by line-shaped grooves extending in a plurality of directions and can facilitate the nitride semiconductor film to grow in a direction perpendicular to each of the grooves, thus achieving reduced density of threading dislocations reaching the active layer. In this way, a light emitting diode with high light emitting efficiency can be manufactured.
Furthermore, a nitride semiconductor substrate having good crystallinity resulting from restricting warping of the substrate is disclosed in Japanese Patent Laying-Open No. 2002-208757. The nitride semiconductor substrate described in Japanese Patent Laying-Open No. 2002-208757 has a surface on which polygonal or circular recesses are formed, and a nitride semiconductor film is grown thereon. In this way, threading dislocations are converged in the respective centers of the recesses to reduce warping of the substrate.